This invention relates to an automatic focus adjusting system and more particularly to an automatic focus adjusting system wherein the focus adjustment is automatically turned off when the object or shooting conditions are not suitable for automatic adjustment. For instance, conditions of low contrast and/or high density of similar subject matter with a deep field of view present difficulties to automatic focusing. Of the many devices for adjusting focus automatically, each has its functional limits. A few specially designed systems work for special purposes, but because of the aforesaid limitations errors can occur in adjusting focus or measuring the subject distance. One reason for such errors comes from the fact that the object to be focused upon is a gathering of several objects which are not placed within the same plane (at right angle with optical axis) and they are not at the same distance from the camera. A small object near the camera appears the same size as a large object which is far away, to automatic focus adjusting devices. The distance to which the objective lens should be focused is not obtainable and confusion results because in such devices the focusing is done by comparing the light from the several objects. Similarly, such devices cannot focus the lens when the amount of light is insufficient or when the object has low contrast or little or no difference in contrast.
Improvements are continuously being made to optical instruments such as cameras having a focusable objective lens. One of the more recent series of improvements is an automatic focusing system for adjusting the focus of the lens to an object distance corresponding to the distance of the camera to remote subjects in the field of view of that lens. Typically, an automatic focusing system may use the principle of spatial image correlation wherein a scene imaged by a first auxiliary optical system is scanned by a sensor and the image therefrom is correlated electronically by another sensor with an image from a fixed auxiliary optical system. Such an automatic focusing system has electronics for evaluating the light bundles from images passed through those optical systems and impinging on sensor arrays. Electronics are used to control power for a motor for driving the focusing lens of an objective in accordance with the relationship of the images transmitted by the optical systems. Focus of the objective occurs when the images from the optical systems are coincident; under such conditions the position of the scanning optical system is related to the position of the objective lens.
Certain of the automatic focusing systems, as described in recently issued United States patents, can be mass produced, are not unduly complex, bulky, and expensive, and are reliable for use in optical instruments or cameras for the mass market. Such systems include a scanning means, which moves an optical axis to sweep an image across a light sensitive array, and which is connected with another similar array of a distance detector. One of the optical axes is fixed relative to the rangefinder and camera lens and the other is moveable so as to scan along the other optical axis. An automatic rangefinder of the type generally described in U.S. Pat. No. 4,002,899 assigned to Honeywell, Inc. and called the Honeywell Visitronics module responds to the two optical systems that bring light from the subject to a detector having a pair of photo sensitive arrays. The module is responsive to the fact that the axes of the optical systems are aligned with one another and directed toward the same subject. Such alignment causes the detector to generate a peak pulse which pulse can be used to adjust the focus of the camera lens. Rangefinding is accomplished by triangulation and comparison of the image from the fixed axis with the image from the scanning axis. By means of suitable electronic circuitry the peak signal can be converted to a control signal for the focusing motor.